F. Eberl, Alcan Rhenalu, Issoire Cedex, France; J. Boselli, Alcoa Incorporated, Alcoa Center, PA; T. Warner, Alcan, Voreppe, France; F. Heymes, Alcan Aerospace, Issoire, France; G. D. Tuss, The Boeing Company, Seattle, WA; V. Dangerfield, Pechiney Rolled Products, LLC, Ravenswood, WV

Summary:

Current alloys for lower wing skin applications are mainly dominated by long durability and high damage tolerance. Generally, 2xxx alloys are used at the T351 temper, which are limited strongly in strength, about 50 ksi is the current typical value. The increase of static properties while maintaining the damage tolerance properties and the combination with high strength stringers for bottom skin applications allows new design approaches, which lead to a beneficial weight reduction. Compared to a 2324 T39 baseline, a new 2x24 will be presented in different tempers. Thanks to an optimized solute content, naturally and artificially aged tempers (T3x and T8x respectively) were possible, so that static properties in the range of 75 ksi could be reached. Even with such high static properties, a low fatigue crack growth rate was obtained due to an optimized grain structure. The optimization of the dispersoid content in combination with the processing selected led to a favorable grain morphology. Laboratory and industrial data illustrating the final properties of the new 2x24 alloy for different tempers will be presented. In order to allow new design approaches, the high strength bottom wing skins should be combined with high strength stringers. Currently 2224 T3511 or 2027 T3511 are the standard. In the following a high strength damage tolerant 7xxx alloy is proposed. The optimization of the major alloying element additions (Zinc, Copper and Magnesium), both in terms of total solute level and in terms of balance, leads to a very high strength solution for damage tolerant stringer sections whilst maintaining appropriate toughness properties. The high properties are achieved by a combination of precipitation hardening and unrecrystallized fibrous grain structure. The corrosion performance depends strongly on aging practice for specifically over-aged tempers. The damage tolerance and fatigue performance is related to a number of metallurgical parameters, including strength, degree of overaging, purity levels, etc. Complementarily to the wing skin data, laboratory and industrial data illustrating the metallurgical principles underlying these properties will be discussed.